Liquids and liquefied gases are stored in closed containers, from which they are transferred into other containers, which in turn may serve as storage containers for transferring into even smaller containers.
In the case of gases liquefied under pressure, removal is effected by means of inherent pressure. In the case of pressure-liquefied gases which consist of more than one component, there is the risk that, depending on the filling level and temperature in the respective container, a gas phase of greater or lesser volume will be produced, which differs from the liquid phase in its proportional composition.
In the case of liquefied gas mixtures, thus the composition of the gaseous or liquid phase may vary according to the temperature of the mixture and the ratio of gas and liquid volumes. When the container level drops, the composition of the mixture in the gas phase and also in the liquid phase thus changes. This change in composition may result in the mixture composition no longer corresponding to the composition of the mixture as originally introduced.
This problem occurs, for example, in the storage of liquefied gases for refrigeration purposes. Both azeotropic mixtures and zeotropic mixtures are used in refrigeration. In zeotropic mixtures the compositions of the gas phase and the liquid phase which is in equilibrium therewith are not identical.
The refrigerant R407C contains HFC32, HFC125 and HFC134a in a weight ratio of 23:25:52 with a tolerance of ±2% by weight per component in accordance with ARI 700/ASHRAE 34/DIN 8960. If this zeotropic mixture is stored in a conventional container and the liquid phase is removed from this container, it is noted that enrichment of HFC134a and depletion of the other two components occurs in the liquid phase. Correspondingly, depletion of HFC134a and enrichment of the other two components occurs in the gas phase. This shift in the composition is undesirable, since associated disruptions may occur in the refrigeration plant, which is set to a given mixture composition. The intended refrigeration performance cannot, for example, be achieved.
JP 8-4997 describes a method with which the changes in concentration when storing liquefied gases and removing them from the storage containers can be avoided. According to this method, either an inert gas or the gas component of the liquefied gas mixture which has the lower boiling point and therefore is enriched in the gas phase is introduced under pressure into the storage container simultaneously with the removal of the gas.